This invention relates generally to programmable read-only memories and more particularly to reprogrammable read-only memories.
As is known in the art, it is frequently desirable to use programmable read-only memory (PROM) devices in many digitally controlled apparatus. In many bipolar PROM devices each memory element includes a fusible link which, when remaining in its closed circuit or nonfused condition, represents one logical state and when blown or open-circuited in response to a programming current represents a second logical state. In other PROM devices the use of an insulating material has been suggested for the memory element, such material having a normally relatively high resistance characteristic representing one logical state, and when programmed by applying a relatively high voltage across the material to breakdown the dielectric characteristic, causes such memory element to have a relatively low resistance characteristic representing a second logical state. When either one of the types of circuits changes the logical state initially represented thereby such change is irreversible and hence, when once programmed, the memory element cannot generally be reprogrammed.